A mobile communication system can be designed to have a hierarchical cell structure wherein a macrocell covering a whole cell is overlaid with microcells (e.g. femtocells) covering parts of the cell. The hierarchical cell structure based on a typical cellular network was proposed to improve frequency resource utilization and to extend cell coverage, for example, communication coverage. When there are geographical obstacles in a cell or a shadowing area is created indoors, the hierarchical cell structure can extend communication coverage. Deployment of a small microcell in a shadowing area where communication is impossible due to signal attenuation enables low cost communication coverage extension. In this case, unused resources are allocated to the microcell in order to prevent interference between the macrocell and the microcell.
As stated above, the hierarchical cell structure can service more UEs and data using frequency resources through efficient use of frequency resources. In an environment where a macrocell is overlaid with microcells, resources used by the macrocell are reused for the small microcells, thereby increasing frequency resource utilization. Consequently, cell capacity can be remarkably increased.
However, interference may occur because a macro UE connected to the macrocell and a micro UE connected to a microcell use the same frequency resources in the spatially non-separated environment. To avert this problem, interference avoidance may be employed.
Interference avoidance is a technique of preventing interference caused by simultaneous transmission from a macro UE and a micro UE. Frequency resource priority is given to the macro UE over the micro UE regarding, and the micro UE is allowed to transmit signals only when the macro UE is not using the frequency resources. Therefore, the micro UE should determine whether the macro UE is transmitting signals or not using the frequency resources. The determination may be made through Cognitive Radio (CR) spectrum sensing.
CR is a technology for wireless communication in which an unlicensed user having no available frequency resources detects temporarily unused frequency resources of a licensed user, referred to as a spectrum hole or white space, and operates adaptively according to an optimum transmission scheme for the detected frequency resources. This frequency sharing scheme was proposed based on the idea that although the licensed user has frequency resources, the licensed user does not use the frequency resources at all times.
Accordingly, the unlicensed user may share the frequency resources with the licensed user without mutual interference by detecting a time period during which the licensed user does not use the frequency resources through spectrum sensing. Spectrum sensing is an essential technique in CR.
Spectrum sensing may be carried out using one of a matched filter detector, a feature detector, and an energy detector in the physical layer of a communication system. The matched filter detector and the feature detector are not popular in view of implementation complexity and the need that an unlicensed user should know signal characteristics of a licensed user in advance. In this context, energy detection-based spectrum sensing will be focused upon herein.
If a macrocell coexists with a microcell through CR spectrum sensing in a hierarchical cell structure, a macro UE and a micro UE serve as a licensed UE and an unlicensed UE, respectively. The microcell is overlaid on the macrocell and the micro UE is connected to the microcell by sensing frequency resources unused by the macro UE.
An environment where a macrocell has available frequency resources and a plurality of macro UEs use the frequency resources simultaneously is considered because of channelization characteristics of a macro UE system whose frequency resources will be reused. For example, an Institute of Electrical and Electronics Engineers (IEEE) 802.16e Mobile Worldwide interoperability for Microwave Access (WiMAX) system divides a channel into 16 subchannels.
When a plurality of micro UEs within a microcell use frequency resources of a macrocell, it is important for the micro UEs to share the frequency resources by detecting a time period during which the frequency resources are not used by the macro UEs through spectrum sensing. However, a method for allocating macrocell frequency resources to each micro UE by a micro Base Station (BS) has yet to be specified.